It is known that, in order to construct a descent and/or approach profile for an aircraft, in particular a transport aeroplane, an FMS-type flight management system of the aircraft defines an optimised vertical profile by performing a backward calculation (i.e., progressing upward in altitude). This backward calculation is carried out on the basis of the runway threshold or, as a function of the approach geometry, from a usual point (such as a “missed approach point” or a “final end point”) up to the cruise flight level (identified by a point TD (“Top of Descent”)), taking account of speed and/or altitude constraints inserted into the flight plan. A deceleration point DECEL is likewise calculated by the FMS system. The point DECEL corresponds to the start of the deceleration towards the approach speed (VAPP). This point DECEL determines the limit between the descent and approach phases.
With this method of backward calculation, the first step is the calculation of the approach profile defined by: a final approach profile, calculated from the runway threshold up to a point FAF (“Final Approach Fix”) or FAP (“Final Approach Point”). This final approach profile is determined in the usual way by a fixed gradient angle, corresponding to the final part defined in the procedure; and an intermediate approach profile from the point FAF/FAP towards the deceleration point DECEL. Along this intermediate profile, the aircraft starts the deceleration from the point DECEL (at the maximum speed, generally 250 knots, or at the lowest speed restriction which can be flown in clean configuration) until the final approach speed (VAPP) generally reached at a height of 1000 feet above the ground.
When the approach profile is defined, the FMS system starts the backward calculation of the descent profile from the point DECEL to the point TD. Taking account of altitude and/or speed restrictions defined in the flight plan, the descent profile is optimised for the reduction of noise and fuel consumption. To this end it includes: a so-called “geometric part” from the point DECEL to the last point where an altitude restriction impacts upon the definition of the descent profile. The geometric part is composed of a sequence of specific constant gradients, which make it possible to meet altitude restrictions. If no altitude restriction impacts on the descent profile, no geometric part is calculated; and a so-called “idle part” from the last restricting altitude restriction or from the point DECEL (if no geometric part exists) to said point TD. This part comprises a so-called “idle” segment along which the aircraft flies between two restrictions with an engine idling speed (which is beneficial for fuel consumption and noise).
The geometric part, if it exists, is composed, from the point DECEL backwards to the last altitude restriction, by a succession of straight segments (so-called geometric segments) defined each time between two points relating to altitude restrictions. This geometric portion is constructed in order to minimise the vertical maneuvers. In particular, if a straight segment makes it possible simultaneously to meet several restrictions, the corresponding part of the vertical profile is constructed by a single straight segment (geometric segment), instead of a plurality of short unaligned straight segments.
In addition, the FMS system usually associates a type with each geometric segment defined in the vertical profile. As a function of the performance levels of the aircraft and the status of the aircraft and external conditions (mass, centre of gravity, altitude, speed, wind conditions and temperature, . . . ) as well as of the gradient of the geometric segment in question, the aircraft exhibits a certain deceleration capacity along said geometric segment. The deceleration capacity defines the type of geometric segment: if the gradient of the geometric segment enables a sufficient deceleration in order that said segment can be flown in clean configuration (that is to say without the slats and flaps extended), said segment is a so-called “clean airbrake”); if the gradient of the geometric segment does not enable a sufficient deceleration in order that said segment can be flown in clean configuration but on the other hand enables it to be flown with “half airbrake”, said segment is a so-called “half airbrake” segment; and if the gradient of the geometric segment does not enable a sufficient deceleration in order that said segment can be flown in clean configuration, even with half airbrake, said segment is called “too steep”.
With the aforementioned usual logic used by the FMS system in order to calculate the profile, as the number of vertical maneuvers is minimised, certain geometric segments may potentially be very long and steep, in particular in environments which impose altitude restrictions far removed from one another (in terms of distance and altitude). These geometric segments do not allow a good deceleration capacity, in particular when they have a considerable gradient. This is one of the reasons for positioning of the point DECEL at high altitude.
The position of the point DECEL (that is to say the starting point of deceleration towards the approach speed) should be able to satisfy operational considerations. The approach should start at an altitude where the aircraft is supposed to start the deceleration towards the approach speed. A point DECEL which is too high is not adapted either to the manner in which the pilots are accustomed to perform the descent and the approach, nor to the speeds expected by the air traffic control at such altitude or distance from the final destination.
In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.